Conventionally, for example, a 4-tandem type color copying machine in which four photoconductive drums are horizontally arranged side by side is known as an image forming apparatus. In such a type of the color copying machine, for example, one polygon mirror is arranged between the second photoconductive drum and the third photoconductive drum.
Such a color copying machine has four light sources used for forming electrostatic latent images on surfaces of respective photoconductive drums. For example, the four light sources are divided into two groups on the left side and the right side with the polygon mirror as the center, each group including two light sources. Each light source emits a laser beam based on an image signal of one color subjected to a color separation process. In order to guide the laser beam from the polygon mirror to each photoconductive drum, a plurality of mirrors functioning as scanning optical systems is arranged.
The laser beams emitted from the two light sources on the left side are reflected by the same reflecting surface of the polygon mirror. The reflected laser beams are scanned in a main scanning direction by the rotation of the polygon mirror and then are guided to the two photoconductive drums on the left side through the scanning optical system. Similarly, the laser beams emitted from the two light sources on the right side are reflected by the same reflecting surface of the polygon mirror.
The reflected laser beams are scanned in the main scanning direction by the rotation of the polygon mirror and then are guided to the other two photoconductive drums on the right side through the scanning optical system. These two laser beams, for example, are incident on the polygon mirror with a opening angle in the main scanning direction. Since the emitted light beam is incident on the polygon mirror directly along the sub-scanning direction, the light source is hardly affected by a change in an optical path due to unevenness of the surface.